This invention relates to ceramic compositions having chemical stability and electronic conductivity at temperatures in the range of 500.degree.-700.degree. C. and more particularly to ceramic compositions useful as cathodes in molten carbonate fuel cells. More specifically, the ceramic compositions have a crystalline structure based on an alkali-metal, transition-metal oxide whose insulating properties have been changed by the substitution of a third metal for a small portion of the alkali metal and/or transition metal in the crystalline structure of the resulting ceramic composition thereby providing favorable electronic conductivity. Various electrode compositions have been previously disclosed for particular fuel cells. U.S. Pat. No. 3,367,802 includes a disclosure of a fuel electrode to be used with an acid electrolyte in a low temperature cell. However, the composition would not be suitable for use at high temperatures in an oxidizing environment and in contact with molten carbonate. U.S. Pat. No. 4,132,619 is also directed to an electrode composition having similar limitations. Disclosures of other compositions are in U.S. Pat. Nos. 3,684,578; 4,203,871 and 4,225,469.
Molten carbonate fuel cells include an anode such as porous nickel, a cathode such as porous lithiated nickel oxide and a molten carbonate electrolyte retained in a porous tile. A representative electrolyte is a mixture of Li.sub.2 CO.sub.3 and K.sub.2 CO.sub.3. In the past, molten carbonate fuel cells of the above type were assembled with a sintered nickel oxide cathode which was lithiated in situ with Li.sub.2 CO.sub.3 in the presence of an oxidant gas (a mixture including O.sub.2 and CO.sub.2) to form lithiated nickel oxide. The lithiation (incorporation of lithium ion in the NiO crystalline structure) of the nickel oxide was important in providing electronic conductivity in the cathode. While many characteristics of the cathode were desirable, it was found that the cathode was not stable in the presence of the molten carbonate electrolyte, and would lose nickel oxide to the electrolyte. It further was observed that the nickel ion in the electrolyte was reduced to nickel metal under conditions at or near the anode. The result was a continuous loss of cathode material. In a similar manner, while some iron oxides were conductive initially and offered some promise as cathode materials, all were found to change in composition during contact with the electrolyte at cell temperatures of 600.degree.-700.degree. C. and change to an essentially nonconductive composition. Under these circumstances, the lithiated nickel and iron oxides did not appear to provide the desired stability plus conductivity during the expected lifetime of 4.times.10.sup.4 hours projected for commercial cells.
In the development of cathode materials for molten carbonate fuel cells, one approach has been to select known conductive materials and test them for chemical stability in the fuel cell cathode environment. Initial results have been less than promising. In general, the known conductive materials do not exhibit a desired combination of chemical stability and electronic conductivity as cathodes at temperatures of 600.degree.-700.degree. C. and in a molten carbonate fuel cell environment.
Accordingly, one object of this invention is to develop ceramic compositions having chemical stability and electronic conductivity at 500.degree.-700.degree. C. A second object of the invention is to develop ceramic compositions useful in forming cathodes for fuel cells. A third object of the invention is to develop a fuel cell cathode for a molten carbonate fuel cell. Another object of the invention is to develop a fuel cell cathode essentially insoluble in molten carbonate electrolyte. A further object is to develop a conductive ceramic for use in other high temperature environments such as MHD equipment. These and other objects will become apparent from the following description.